1. Field of the Invention
The present invention relates to an optical device using a planar lightwave circuit, and more particularly to a photomask used in the manufacture of an optical fiber block adapted to connect an optical fiber to a planar lightwave circuit.
2. Description of the Related Art
Planar lightwave circuits have been frequently used in processing of optical signals. For example, such planar lightwave circuits may be used for the splitting, modulation, switching, and multiplexing of optical signals. In association with such planar lightwave circuits, a number of research efforts have been made to provide an integrated chip in which a waveguide is formed on a planar substrate. In order to fabricate such a planar lightwave circuit chip, it is necessary to use techniques for designing and fabricating a waveguide as well as and packaging techniques. Such planar lightwave circuit chips use optical fiber blocks so that it is coupled with an optical fiber. Such optical fiber blocks are adapted for multiple access. In an optical fiber block, V-grooves are formed at a silicon substrate in accordance with the intrinsic wet etching characteristics of the silicon substrate. Advantageously, such an optical fiber has many applications because its V-grooves can have an accurate pitch.
FIG. 1 illustrates a typical planar lightwave circuit, and optical fiber blocks aligned with and bonded to the planar lightwave circuit. The alignment and bonding of the planar lightwave circuit and optical fiber blocks will be described in conjunction with FIG. 1. In the alignment and bonding structure of the conventional planar lightwave circuit shown in FIG. 1, input and output optical fiber blocks 20 and 30 are arranged at opposite sides of a planar lightwave circuit 10 while being aligned with each other. The input optical fiber block 20 serves to connect a single optical fiber F1 to an input port of the planar lightwave circuit 10. The output optical fiber block 30 serves to connect an optical fiber ribbon F2 to an output port of the planar lightwave circuit 10. In this arrangement, the planar lightwave circuit 10 is connected to the single optical fiber F1 and the optical fiber ribbon F2 by the input and output optical fiber blocks 20 and 30, respectively. The aligned state of the planar lightwave circuit 10 with the input and output optical fiber blocks 20 and 30 is maintained by an adhesive Bo such as epoxy resin.
Referring to FIG. 2, the structure of a photomask used to fabricate one of the input and output optical fiber blocks 20 and 30 will be described. As shown in FIG. 2, a photomask 40 is used to fabricate an optical fiber block in accordance with a photolithography process. The photomask 40 includes two portions, a laser transmission portion BL that allows laser beams to pass therethrough, and a laser shield portion WH that prevents laser beams from passing therethrough. The laser transmission portion BL corresponds to a black portion in FIG. 2. The laser shield portion WH corresponds to a white portion in FIG. 2.
The photomask 40 is divided into two sections, a first section 410 for an optical fiber alignment region of an associated optical fiber block (shown in FIG. 3 or 4), and a second section 420 for a stress-reducing region. The first section 410 is provided with a plurality of slit arrays 401, 402, and 403 each having a desired slit pitch, and portions other than the slit pitches of those slit arrays, that is, portions 405, 406, 407, and 408. In particular, the photomask 40 has a maximum pitch at th& opposite end portions 405 and 406 of the first section 410. The portions 407 and 408 arranged among the slit arrays 401, 402, and 403 have a pitch smaller than that of the end portions 405 and 406. The photomask 40 is adapted for a 24-core optical fiber block.
FIGS. 3 and 4 illustrate an optical fiber block 50 fabricated using the conventional photomask 40 along with a photolithography process. The optical fiber block is an 8-core optical fiber block. The optical fiber block 50 includes an optical fiber alignment region 510, and a stress-reducing recessed region 520. The optical fiber alignment region 510 is provided with a plurality of V-groove arrays 501 each having a plurality of V-grooves 501a at which bare fiber portions of an optical fiber ribbon are arranged. A planar partition portion 503, which has a pitch different from that of the V-grooves 501a, is arranged between adjacent ones of the V-groove arrays 501. Planar end portions 502, which have a pitch different from that of the V-grooves 501a, are also provided at opposite ends of the optical fiber alignment region 510, respectively (only one planar end portion 502 is shown in FIG. 3).
The planar portions 502 and 503 of the optical fiber alignment region 510 have a pitch different from that of the V-grooves 501a and are inevitably formed with protrusions A and B at an interface between the optical fiber alignment region 510 and the stress-reducing recessed region 520. The protrusions A and B extend from the optical fiber alignment region 510 to the stress-reducing recessed region 520. The protrusions A and B are inevitably formed due to the intrinsic crystalline structure of the optical fiber block along with differences in etching rate and etching selectivity exhibited in the optical fiber block during a wet etching process.
An optical fiber ribbon (not shown) is then laid on the optical fiber block 50, fabricated as described above, in an aligned state. In this state, epoxy resin is applied to the optical fiber block 50. The applied epoxy resin flows in directions indicated by the arrows • and • in FIG. 3. As shown in FIGS. 3 and 4, the interface between the optical fiber alignment region 510 and the stress-reducing recessed region 520 has a non-uniform protrusion shape at the portions 503 having a pitch different from that of the V-grooves 501a, and the opposite end portions 502, due to the wet etching process. An enlarged photograph of the protrusions A and B of the optical fiber block is shown in FIG. 5.
The protrusions A and B interfere with the flow of epoxy resin used in a bonding process. In particular, the epoxy resin may be collected at the portions 503 having a pitch different from that of the V-grooves 501a. Furthermore, there are problems in that the optical fiber mounted to the optical fiber block may be subjected to stress, or broken due to a difference in the amounts of epoxy resin respectively applied at different sites for multiple bonding, or a difference in the shrinkage of epoxy resin at different sites when the epoxy resin is cured. Where stress is generated at the optical fiber ribbon, optical loss may occur. In severe cases, the optical fiber may be broken.